The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known spark-ignition (SI) engines introduce an air/fuel mixture into each cylinder which is compressed in a compression stroke and ignited by a spark plug. Known compression ignition engines inject pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke which ignites upon injection. Combustion for both gasoline engines and diesel engines involves premixed or diffusion flames controlled by fluid mechanics.
SI engines can operate in a variety of different combustion modes, including a homogeneous SI combustion mode and a stratified-charge SI combustion mode. SI engines can be configured to operate in a homogeneous-charge compression-ignition (HCCI) combustion mode, also referred to interchangeably as controlled auto-ignition (HCCI) combustion, under predetermined speed/load operating conditions. The controlled auto-ignition (HCCI) combustion includes a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry. An engine operating in the controlled auto-ignition (HCCI) combustion mode has a cylinder charge that is preferably homogeneous in composition, temperature, and residual exhaust gases at intake valve closing time. Controlled auto-ignition (HCCI) combustion is a distributed kinetically-controlled combustion process with the engine operating at a dilute air/fuel mixture, i.e., lean of an air/fuel stoichiometric point, with relatively low peak combustion temperatures, resulting in low nitrous oxides (NOx) emissions. The homogeneous air/fuel mixture minimizes occurrences of rich zones that form smoke and particulate emissions.
Controlled auto-ignition (HCCI) combustion depends strongly on factors such as cylinder charge composition, temperature, and pressure at intake valve closing. Hence, the control inputs to the engine must be carefully coordinated to ensure auto-ignition combustion. Controlled auto-ignition (HCCI) combustion strategies may include using an exhaust recompression valve strategy. The exhaust recompression valve strategy includes controlling a cylinder charge temperature by trapping hot residual gas from a previous engine cycle by adjusting valve close timing. In the exhaust recompression strategy, the exhaust valve closes before top-dead-center (TDC) and the intake valve opens after TDC creating a negative valve overlap (NVO) period in which both the exhaust and intake valves are closed, thereby trapping the exhaust gas. The opening timings of the intake and exhaust valves are preferably symmetrical relative to TDC intake. Both a cylinder charge composition and temperature are strongly affected by the exhaust valve closing timing. In particular, more hot residual gas from a previous cycle can be retained with earlier closing of the exhaust valve leaving less room for incoming fresh air mass, thereby increasing cylinder charge temperature and decreasing cylinder oxygen concentration. In the exhaust recompression strategy, the exhaust valve closing timing and the intake valve opening timing are measured by the NVO period.
In engine operation, the engine airflow is controlled by selectively adjusting position of the throttle valve and adjusting opening and closing of intake valves and exhaust valves. On engine systems so equipped, opening and closing of the intake valves and exhaust valves are accomplished using a variable valve actuation system that includes variable cam phasing and a selectable multi-step valve lift, e.g., multiple-step cam lobes which provide two or more valve lift positions. In contrast to the throttle position change, the change in valve position of the multi-step valve lift mechanism is a discrete change, and not continuous.
When an engine operates in a controlled auto-ignition (HCCI) combustion mode, the engine control includes lean or stoichiometric air/fuel ratio operation with the throttle wide open to minimize engine pumping losses. When the engine operates in the SI combustion mode, the engine control includes stoichiometric air/fuel ratio operation, with the throttle valve controlled over a range of positions from 0% to 100% of the wide-open position to control intake airflow to achieve the stoichiometric air/fuel ratio.
When an engine operates in a controlled auto-ignition (HCCI) combustion mode including recycling of exhaust gas using a variable valve actuation system, the auto-ignited combustion depends on the temperature, composition, and pressure of the cylinder charge, including a large portion of the cylinder charge being residual gas at intake valve closing. For example, with the exhaust recompression strategy, the cylinder charge temperature is controlled by trapping the hot residual gas from the previous engine cycle by closing the exhaust valve early during the exhaust stroke, while opening the intake valve at a late timing symmetrical to the exhaust valve closing timing. The cylinder charge composition and temperature depend on how early the exhaust valve closes during the exhaust stroke. When the exhaust valve closes earlier during the exhaust stroke, a greater amount of the hot residual gas from previous engine cycle is trapped in the cylinder, thereby increasing the cylinder charge temperature.
The amount of the residual gas trapped in the combustion chamber during operation in the spark ignition SI combustion mode is relatively small, and therefore the effect of the residual gas temperature on the amount of the fresh air charge is insignificant. When an engine operates in a controlled auto-ignition (HCCI) combustion mode, the cylinder charge contains a significant amount of hot residual gas, which can substantially affect the amount of incoming fresh air charge. This is because when the large amount of hot residual gas is compressed during recompression, a significant amount of heat is transferred to the cylinder wall as the piston moves toward top-dead-center. Thus, when the intake valve opens at a late timing symmetrical to the exhaust valve closing timing, the pressure of the residual gas is significantly lower than when the exhaust valve closes, creating a vacuum in the cylinder, promoting added fresh air entering the cylinder. However, when there is incomplete combustion or no combustion in one or more cylinders, e.g., as a result of a misfire, partial-burn or fuel cutoff event (e.g., during a deceleration event), the temperature of the residual gas can be significantly lower. In such engine operation, the amount of heat transfer is minimal and the vacuum created in the cylinder when the intake valve opens is less. Thus, the engine mass airflow decreases when there is incomplete combustion or no combustion in one or more of the cylinders.
Known powertrain control architectures include mass airflow metering devices for monitoring intake airflow to the engine. One embodiment of a mass airflow metering device uses anemometry to determine the intake air mass flowrate. Known anemometric measurement systems include placing a sensing device having electrical resistive properties correlative to mass airflow in the intake air flowstream. The sensing device can be incorporated into a known electrical circuit that includes measurement and signal conditioning to measure electrical current flow thereacross. There is a correlation between the electrical current flow across the sensing device and the mass airflow past the sensing device that can be measured and calibrated using the electrical circuit. The electrical circuit communicates the mass airflow to a control module. Embodiments of anemometric sensing systems can include different sensing devices, e.g., hot-wire and hot-film, and different electrical circuits.
Known powertrain control architectures include systems for monitoring operation of the mass airflow metering device to ensure proper engine control and operation, and to identify component and system faults. Monitoring requirements for a mass airflow metering device include monitoring to detect electrical shorts and open circuits. Monitoring requirements for a mass airflow sensor include monitoring to detect in-range flow rationality, wherein a signal output from a mass airflow metering device is monitored for signal bias or unexpected variations. Known in-range faults for a mass airflow metering device can be caused by faults that affect resistance(s) of elements of the electrical circuits, faults that change the resistance of the sensing device, and faults that change aspects of the sensing device that interfere with its capacity to meter incoming air, such as dust buildup on the sensing device and changes related to its orientation in the air flowstream.